This application claims priority to International Application No. PCT/DE00102436 which was published in the German language on Jul. 19, 2000.
The invention relates to a circuit arrangement for operation of a relay, and in particular, to a current having a timer unit which provides a relay switching-on current for a time which is predetermined by the timer unit and which provides a relay holding current, which is less than the relay switching-on current for a subsequent holding period.
U.S. Pat. No. 5,107,391 discloses a circuit arrangement in which the current flowing through at least one relay (i.e. its field coil) is controlled by means of an electronic switch in the form of a field-effect transistor. When it is switched on, the relay is supplied with a holding current throughout its holding period. The magnitude of the holding current is governed by the duty ratio of electrical pulses which drive the electronic switch. The temperature of the relay is measured by means of a temperature sensor, and the voltage which is applied to the field coil of the relay is measured by means of a voltage sensor. These measurement variables, as well as information which is stored in a function memory and relates to the nominal values of the relay, are used to define the duty ratio of the pulses, and thus to define the magnitude of the holding current. The relay nominal values must therefore be known in order to properly operate the circuit. A voltage change which occurs on the field coil of the relay is used to identify that the relay has been switched on, and causes a timer unit to be started. This timer unit uses a continuous pulse to switch on the electronic switch, so that a switching-on current flows which is sufficient to ensure that the relay is switched on. Once the time which is predetermined by the timer unit, and which must be longer than the time required for the relay that is being used to be switched on has elapsed, the continuous pulse ends, and the holding current, which is governed by the duty ratio of the pulses, still flows through the relay.
In order to ensure that a sufficient holding current flows to hold the relay in the switched-on state, the circuit takes account of the voltage across the field coil, the temperature and the nominal values for the relay. Individual discrepancies from the nominal values for the relay, in particular discrepancies in the coil resistance, are ignored, however. Discrepancies such as these can occur, for example, during the manufacture of the relay, due to aging processes during operation, or due to oxidation of conductors and contacts of the field coil.
Furthermore, although the circuit is able to take account of fluctuations in the operating voltage by measuring the voltage across the field coil of the relay, the circuit is, however, intended for operation from a voltage source with a predetermined nominal voltage, for example from a motor vehicle battery with a voltage of 12 V. Circuits with components of different sizes are therefore required to operate relays from different operating voltages.
One embodiment of the invention discloses a circuit arrangement for operation of a relay. Discrepancies in the relay from its nominal values have no effect on the magnitude of the switching-on current and holding current, and need not be taken into account with regard to the size of the components in the circuit. Furthermore, it is possible to use the cireuit, with one and the same relay, from operating voltages of different magnitude.
In this embodiment, the relay switching-on current and the relay holding current are constant currents, which are supplied from at least one constant current source. A constant current is thus used both as the relay switching-on current and as the relay holding current, whose magnitude is not influenced either by discrepancies in the relay from its nominal values or by operation of the cireuit arrangement from operating voltages of different magnitude.
A constant current source whose constant current magnitude is variable can be used to supply the constant relay switching-on current and the constant, lower relay holding current. The constant current source supplies the relay switching-on current from the start of the switching-on process. Once the relay has been switched on and the time which is predetermined by the timer unit has elapsed, the constant current is reduced to the relay holding current.
In another embodiment of the invention, the cireuit arrangement includes, for example, a first constant current source which provides the relay holding current during the time which is predetermined by the timer unit and during the subsequent holding period, and a second constant current source which provides a constant current, which is superimposed on the relay holding current in order to form the relay switching-on current, during the time which is predetermined by the timer unit. This has the advantage that relatively simple constant current sources can be used, whose constant currents need not be variable.
In one aspect of the invention, a switch, which is closed during the time that is predetermined by the timer unit, can advantageously be located in the current path of the constant current provided by the second constant current source. Once this time has elapsed, the switch is opened, thus making it easy to switch between the relay switching-on current and the relay holding current.
In another aspect of the invention, the timer unit and/or the switch may require an auxiliary voltage. A voltage drop across at least one electrical component that is connected in series with one of the constant current sources can advantageously be used for this auxiliary voltage.
In one embodiment, a resistor may be used as the electrical component. Since the current flowing through the resistor is constant, the voltage which is dropped across this resistor is also constant, and may be used as an auxiliary voltage.
In another embodiment, a zener diode can advantageously be used as the electrical component. A zener diode has the advantage that the voltage drop which occurs across it is constant even when the current flowing through the zener diode changes. By way of example, a situation may arise during operation of a relay which requires a change to the switching-on current and/or holding current, and hence appropriate changes to the constant currents. The advantage just mentioned is also applicable when series-connected diodes are used as electrical components.
In the circuit arrangement according to the invention, both constant current sources may, for example, supply a constant current of the same magnitude. In this case, the relay switching-on current is twice the magnitude of the relay holding current.